The American Lung Association’s (ALA) 27th annual State of the Air report grades counties for unhealthy levels of ground-level ozone and particle pollution. It found that 33.5 million, or 46 percent, of children are exposed to unhealthy air pollution levels. Because their lungs are still developing, kids breathe more air relative to their size than adults. Long-term pollution exposure in youngsters can lead to reduced lung growth, asthma, respiratory diseases, impaired cognitive functioning, heart attacks and cancer.

While the U.S. had made progress in reducing air pollution, the ALA warns that these advances are threatened by extreme heat, wildfires and the rollback of air pollution standards by the U.S. Environmental Protection Agency.